Quote:
Originally Posted by John L Stewart
[8. To do the above one needs to degenerate the first stage gain, rather than
to use a Partick's attenuator after it. To degenerate the 1-st stage gain
you need to place say 22K resistor in series with the 1-st stage cathode,
and shunt this resistor by a 0.22...0.47uF capacitor. Thus for medium
frequencies the 1-st stage will be working as usual, but ay low frequencies
the transconductance will be degenerating with the perfect linearity of the
first stage maintained. It is the same shelving, but implemented in a wise
linear mode. (Of course the grid leak can not be taken to GND any more, it
should be connected to the cathode or a tap in this 22K resistor.) At
infinitely low frequencies the gain of the first stage is to be degenerated
to 10...20 so that the 1-st stage output is just below the negative bias of
6AQ5 and the later is never overloaded.
Usually I apply mods up to #7 and sometimes #8 too if I can find enough free
solder lugs around the 1-st audio stage tube. Of course, all of the above
implies the cathode of the 1-st audio tube has to be free of duo-diode
functionality. That forces to use separate diodes for AGC and the AM
detector. In some cases instead of 6Q7, 6B8, 6AV6, etc. I might use a 6SL7
tube with one triode as the 1-st audio and the other triode as a diode for
the AM detector, and a silicon diode as an AGC detector.
Regards,
Alex
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Here it is again with the 1st AF grid leak corrected. The NFB network should probably be conjugate with the 1st AF cathode network but that gets this one to a 3 microfarad NFB cap.
But otherwise would be OK.
Cheers, John[/quote]
Hello Again Alex- I checked this cct again & find the cutoff begins at an f1 about 10X too high using the CR network you recommended, 0.22 Ck & 22K Rk. Have a look at Fig. 12.3B, p484 in RDH4 for guidance. The CR time constant appears to predict a point well down on the curve, not the -3db point.
Your proposed cct fix still looks quite useful for the application discussed.
Cheers, John
Cheers, John